Mechanization and motorization have steadily increased the emission of noise. Consequently, depending upon the time and place, the effects of noise have reached or exceeded tolerable limits. The control or reduction of noise has become an important objective. The most significant sources of noise are traffic on roads, railways, in the air, industry, building, and in sport such as shooting ranges.
It is known to erect protective devices directly at the point of origin for the noise as an effective measure for the reduction of the noise. For example, protective walls or protective mounds may be set up alongside roads. Their effectiveness is conditional and limited. High buildings may be protected only on the lower floors depending upon the distance the buildings are from the road. Noise also passes over the protection devices with the result that only a restricted noise shadow zone exists behind the devices. In certain circumstances, unprotected zones may be subjected to increased noise because of sound reflections. In many cases, however, the protection afforded by using sound absorbing materials is sufficient because a proportion of the noise is destroyed.
Significant or complete protection of the environment against traffic is possible. The road may be laid underground, sunk below the level of the surrounding terrain and covered over, or lined on all sides. The disadvantages of these methods, however, lies in the high construction and operating costs. Artificial ventilation and lights are necessary. Even with high protective walls on both sides, ventilation and lighting present a problem.